Question

This code…

#include <iostream>

int main(int argc, char * argv[])
{
    char c = 'A';
    std::cout << &c;
    return 0;
}

…correctly outputs “A” both in Eclipse debug mode and on the command line.

However when I modify the code to…

#include <iostream>

int main(int argc, char * argv[])
{
    char c = 'A';
    bool b = true;
    std::cout << &c;
    return 0;
}

…it outputs “A␁” (the latin letter ‘A’ followed by the ‘start of header’ ASCII control character) in Eclipse debug mode and on the Windows 7 command line. Incidentally, when using bool b = false instead I don’t get the ␁ character.

I know ␀ has the value 0, and ␁ has the value 1, but why is cout << &c affected by the bool? Can anyone explain why this is?

Edit: forgot to add my environment: Windows 7 64-bit, MinGW with g++ 4.5.2, in Eclipse Indigo

Answer 1

In the first version you are getting (un)lucky.

In the second one you are getting (un)lucky (that it does not crash) but it prints more garbage.

When you use & on a char object you get a char*. When you try and stream a char* it acts differently to all other pointers (which normally prints the address). But a char* is assumed to be a C-String. A C-String is a sequence of bytes terminated by a null character ‘\0’;

What is actually happening is that it is printing every memory location (treating it as a char) starting at the variable ‘c’ and moving through memory until it finds the character ‘\0’

Example 1 above:

In your case you are lucky there happens to be a null character lying around in memory just after the variable ‘c’.

Example 2 above:

In your case you are lucky there happens to be a null character lying around in memory just after the variable ‘b’. But the variable ‘b’ is also be interpreted.

• When the value of ‘b’ is false: converted to a char this is ‘\0’ (same thing). So only the letter ‘A’ is printed.
• When the value of ‘b’ is true: converted to a char this is ‘\01’. Not ‘\0’ so it is printed. You are then lucky that the next byte is a ‘\0’ and the printing stops.

What you actually wanted to do was:

std::cout << c; // prints a character

Or you could have created a string:

char const* c = "A";  // Creates a null terminated C-String. Note the double quotes " "

Or to get the address

std::cout << static_cast<void*>(&c); // All other pointer types will print the address
                                     // Of the standard types only char* is treated 
                                     // differently with its own overload.